1. Field of the Invention
The invention relates to page-based optical data storage. The invention further relates to the readout of phase-modulated signals in page-based optical data storage applications.
2. Background Art
In page-based optical data storage, a page of data is read by imaging a two-dimensional data page onto a two-dimensional detector. The detector detects the image, and the image is subsequently processed to decode the data.
Page-based optical data storage technologies and, in particular, holographic data storage (HDS), have benefited greatly from the recent development of CMOS active pixel sensors (also known as APS detectors), such as the ones available today in high resolution digital cameras. These sensors, compared to traditional charge-coupled device (CCD) detectors, present the advantages of lower unit cost (due to the simpler design and manufacturing) and design flexibility provided by the CMOS process, allowing more functionality to be included in the detector chip. However, these sensors have invariably been optimized for consumer imaging and machine vision applications, not for data storage.
Most page-based optical data storage approaches currently pursued use amplitude modulation of the optical wavefront (i.e., on-off pixels). Significant improvements in storage density and BER can be achieved by using phase modulation (e.g., 0 to π phase modulation of the pixels followed by coherent PSK detection). Some of the advantages are: higher dynamic range (e.g., in holographic data storage, signal power falls as 1/M instead of 1/M2, where M is the number of the overlapped holograms, typically about 102), very low intensity of the DC spot in the focal plane (translating into more efficient use of the available dynamic range and lower chance of damage to the material), heterodyne gain at detection, 3-dB gain in SNR and common-mode noise rejection by using differential detection, lower BER for the same SNR by using coherent PSK detection (compared to incoherent ASK), and lower crosstalk between neighboring pixels and between collocated pages, due to noise cancellation.
These advantages are provided at the cost of higher hardware complexity and higher sensitivity to alignment.
Background information may be found in Paulo E. X. Silveira, Optoelectronic Signal Processing Using Finite Impulse Response Neural Networks, Ph.D. dissertation, University of Colorado, 2001; Geoffrey W. Burr, Holographic Storage, Encyclopedia of Optical Engineering, Marcel Dekker, Inc., 2003; Nicolas Blanc, CCD versus CMOS—has CCD imaging come to an end?, Photogrammetric Week ‘01’, D. Fritsch and R. Spiller Eds. Wichmann Verlag, Heidelberg, 2001; Jean Schleipen et al., Optical Heads, Encyclopedia Of Optical Engineering, Marcel Dekker, Inc., 2003; and Simon Haykin, Digital Communications, John Wiley & Sons, 1988.
For the foregoing reasons, there is a need for an improved sensor that is optimized for phase detection in page-based optical data storage and that addresses the challenges posed by heterodyne phase detection.